feat: add checkoutButton title to PaymentSheetConfiguration - #63

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@2f8e54208210a422b2efd51efaa6bd6d7ca8920f, actions/checkout@f43a0e5ff2bd294095638e18286ca9a3d1956744, actions/setup-node@3235b876344d2a9aa001b8d1453c930bba69e610. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/